This invention relates to a method of beneficiating catalyst used in fluid catalytic cracking operations at petroleum refineries used to convert heavy hydrocarbon oils into lighter fractions and especially into fractions containing high concentrations of gasoline and other liquid hydrocarbon fuels.
In general, gasoline and other liquid hydrocarbon fuels boil in the range of about 100.degree. to about 650.degree. F. However, the crude oil from which these fuels are made contains a diverse mixture of hydrocarbons and other compounds which vary widely in molecular weight and therefore boil over a wide range. For example, crude oils are known in which 30 to 60% or more of the total volume of oil is composed of compounds boiling at temperatures about 650.degree. F. Among these are crudes in which about 10% to about 30% or more of the total volume consists of compounds so heavy in molecular weight that they boil above 1025.degree. F. or at least will not boil below 1025.degree. F. at atmospheric pressure.
Because these relatively abundant high boiling components of crude oil are unsuitable for inclusion in gasoline and other liquid hydrocarbon fuels, the petroleum refining industry has developed processes for cracking or breaking the molecules of the high molecular weight, high boiling compounds into smaller molecules which do boil over an appropriate boiling range. The cracking process which is most widely used for this purpose is known as fluid catalytic cracking (FCC). Although the FCC process has reached a highly advanced state, and many modified forms and variations have been developed, their unifying factor is that a vaporized hydrocarbon feedstock is caused to crack at an elevated temperature in contact with a cracking catalyst that is suspended in the feedstock vapors. Upon attainment of the desired degree of molecular weight and boiling point reduction, the catalyst is separated from the desired products.
Crude oil in the natural state contains a variety of materials which tend to have quite troublesome effects on FCC processes, and only a portion of these troublesome materials can be economically removed from the crude oil. Among these troublesome materials are coke precursors (such as asphaltenes, polynuclear aromatics, etc.), heavy metals (such as nickel, vanadium, iron, copper, etc.), alkali metals (such as sodium, potassium, etc.), sulfur, nitrogen and others. Certain of these, such as the alkali metals, can be economically removed by desalting operations, which are part of the normal procedure for pretreatinq crude oil for fluid catalytic cracking. Other materials, such as coke precursors, asphaltenes and the like, tend to break down into coke during the cracking operation, which deposits coke on the catalyst, impairing contact between the hydrocarbon feedstock and the catalyst, and generally reducing its potency or activity level. The heavy metals transfer almost quantitatively from the feedstock to the catalyst surface.
As the catalyst is reused again and again for processing additional feedstock, which is usually the case, the heavy metals can accumulate on the catalyst to the point that they unfavorably alter the composition of the catalyst and/or the nature of its effect upon the feedstock. For example, vanadium tends to form fluxes with certain components of commonly used FCC catalysts, lowering the melting point of portions of the catalyst particles sufficiently so that they begin to sinter at cracking temperatures and become ineffective cracking catalyst. Accumulations of vanadium and other heavy metals, especially nickel, also "poison" the catalyst. They tend in varying degrees to promote excessive dehydrogenation and aromatic condensation, resulting in excessive production of carbon and gases with consequent impairment of liquid fuel yield. An oil such as a crude or crude fraction or other oil that is particularly abundant in nickel and/or other metals exhibiting similar behavior, while containing relatively large quantities of coke precursors, is referred to herein as a carbo-metallic oil, and represents a particular challenge to the petroleum refiner.
Since the various heavy metals are not of equal catalyst poisoning activity, it is convenient to express the poisoning activity of an oil containing a given poisoning metal or metals in terms of the amount of a single metal which is estimated to have equivalent poisoning activity. Thus, the heavy metals content of an oil can be expressed by the following formula (patterned after that of W. L. Nelson in Oil and Gas Journal, page 143, Oct. 23, 1961) in which the content of each metal present is expressed in parts per million of such metal, as metal, on a weight basis, based on the weight of feed: ##EQU1## According to conventional FCC practice, the heavy metal content of feedstock for FCC processing is controlled at a relatively low level, e.g., about 0.25 ppm Nickel Equivalents or less. The present invention is concerned with the processing of feedstocks containing metals substantially in excess of this value and which therefore have a significantly greater potential for accumulating on and poisoning catalyst.
The above formula can also be employed as a measure of the accumulation of heavy metals on cracking catalyst, except that the quantity of metal employed in the formula is based on the weight of catalyst (moisture free basis) instead of the weight of feed. In conventional FCC practice, in which a circulating inventory of catalyst is used again and again in the processing of fresh feed, with periodic or continuing minor addition and withdrawal of fresh and spent catalyst, the metal content of the catalyst is maintained at a level which may for example be in the range of about 2000 to about 6000 ppm Nickel Equivalents. The process of the present invention is concerned with the use of catalyst having a substantially larger metals content, and which therefore has a much greater than normal tendency to promote dehydrogenation, aromatic condensation, gas production or coke formation. Therefore, such higher metals accumulation is normally regarded as quite undesirable in FCC processing.
Presently, the majority of FCC operations utilize a dual fluidized beds of the type described and shown in Modern Petroleum Technology, 4th edition, Hobson & Phol, pgs. 288-309, the disclosure of which is incorporated herein by reference.
The Hettinger, Jr. et al. U.S. Pat. No. 4,406,773, issued Sep. 27, 1983, particularly FIGS. 2 and 3 thereof show typical dual fluidized beds capable of carrying out the process of the invention. Heating of the catalyst, typically about 55 microns in diameter, is necessary to carry out the primary endothermic reactions that form the network of cracking reactions. Oil charge undergoes cracking to form light weight products. Because the bed is fluidized, a perfectly mixed equilibrium population of catalyst exist in the reactor. Undesirable secondary reactions such as olefin polymerization and cracking contribute to coking the catalyst surface. Coking chokes off efficient catalytic activity. At the same time, metals build up on the catalyst is responsible for "aging" and "poisoning" the catalyst. Dehydrogenation and high coke formation typically occurs when the catalyst is poisoned by metals such as nickel, vanadium and iron. Some empirical work suggests that nickel is more than four times as problematical as the vanadium and iron in catalyst poisoning, which is why the formula set forth above has been used extensively in the fluid catalytic cracking art.
Coking and the need for the catalyst to supply the thermal demands of cracking requires the operators to constantly withdraw a catalyst stream and send it to a second fluidized bed in the Regenerator. Combustion air in the Regenerator burns coke off the catalyst, and heats the catalyst so that it comes to the temperature required for optimal cracking operation. Maintaining this heat balance between the reactor and regenerator units is an important consideration in the design of the units. The coking rate of an individual catalyst particle increases as the catalyst ages and acquires metal poisons. Excessive "coking" means that the lay down rate of coking on the catalyst exceeds that needed to reheat the catalyst in the Regenerator. The typical operating procedure at a refinery is to replace a few percent of the catalyst inventory each day with a fresh charge to maintain an equilibrium age and population in the catalyst.
The Hettinger, Jr. et al. patent teaches the use of a magnetized steel mesh to attract zeolite containing catalyst which are poisoned with the metals previously identified. Hettinger, Jr. et al. teach the application of a magnetic field to steel mesh in the reaction zone of a fluidized bed where the more magnetically attracted particles are attached to the steel mesh and are thereby removed from the catalyst leaving those catalyst with a lesser magnetic attraction.
There are a number of problems with the Hettinger, Jr. et al. approach. Principally, the difficulty with the Hettinger, Jr. et al. approach is that it is based on the assumption that the higher parts per million of nickel equivalence on the catalyst, the less effective the catalyst is in providing a large proportion of gasoline in the cracking operation. Moreover, the industry since the Hettinger, Jr., et al. patent has accepted the teaching of the patent that the greater the nickel equivalence in ppm the more poisoned the catalyst and the worse the catalyst is in providing gasoline and other desirable products in the cracking process.
It has been discovered that there is anomaly in the relationship between the concentration of nickel equivalent poisoning and the production of gasoline from cracking operation, wherein some catalyst having a higher degree of nickel poisoning actually produces more gasoline than catalyst with lower concentrations of nickel equivalents.